Push-pull amplifier control



May 7, 1935. w. s. BARDEN PUSH-PULL AMPLIFIER CONTROL Filed Feb. l2, 1952 wlLLlAM s. BARDEN Patented May 7, 1935 d t UNITED A STATES oFFlcE" Williamstoddara Berden, Stapleton, N.` Y., as-

signor to Radio Corporation of America, a cor-r poration of vDelaware Application February 12, 1932,l 'serial No. 5927,4601 A.

f 12 claims (ol. 179-171) My present invention relates to push-pull ampliacteristic of myy invention are set forth in parier circuits, and more particularly to push-pull ticularity in the appended claims, the inven-` circuit arrangement for gain control. f tion'itself, however, as to both its organization As is well known the ideal push-pull amplifier and method of operation will best be understood should be so constructed that the gridcircuits by reference to ther following description taken-in 5 of both tubes are balanced at all times with reconnection withithe drawing in which I have inspect to ground. This ideal design is not easily dicated diagrammatically one -circuit organiza-A secured when it is desired to incorporate in the tion whereby my invention may be carried into push-pull circuit a simple gain control arrangeeifect. j y

ment. Hence, it may be stated that it is one of In the drawing,

the main objects of my present invention to pro- Fig. 1 diagrammatically shows a radio receiv- Vide a push-pull amplifier circuit, wherein 'the ing circuit embodying.- the presentiinvent'ion, tubes employed are preferably of l the 4pentode `Fig.2 graphically illustrates the operation `of the type, which amplifier includes in its input circuit audio frequency amplifier of Fig. 1 as a class B an extremely simple type of gain control arrangeamplifier. I i 15 ment comprising but a single adustable element, Considering, now,I Fig. 1 it will be .noted that the grid circuits of the amplifier tubes being the reference numeral l designates asource'of so designed that they areV balanced at all times Signalenelgy whichrv is adapted 'G0 Supply v'radio with respect to ,ground` frequency energyv to a multi-stage amplifier ycon- Those skilled in the prior art are, additionally, ventionally represented by the numeral 2. The 2o well acquainted with the fact that an amplifier amplified energy; from the ampliel 2 iS lChen circuit may be adjusted to'function as a class A detected in a detector circuit embodying an elecamplifier, the .grid` having a direct 'current potIOndSehaIge JCube 3 having S grid Cireutartential such that it is biased toY a linear portion ranged in the Well known manner'fondetection by er, the grid having a direct current potential It' will berecognized lthtuthe afOIegOiIlg elesuch that it is biased at the cut-offportion ments @emprise the Conventional elements of a of the aforementioned characteristic; or,as a modem ITadO VI'eCeVeIQaduihelefOleV need not class Camplier wherein the grid is biased at a be deSeI'bed in any further detail.

more negative point than the aforementioned The deteeted Output frein the detector Circuit 30 cnt off point, is impressed upon they, common input circuit of The present invention including the simple and theV push-pull' audio frequency amplifier by highly effective type of gain control is-not only means 0f an audio frequency coupling` transwell adapted for the class A and C pushL-pull formerl, the primary 5 of which transformer amplifier arrangements, but is particularly suited has connected across it a resistor 6.- having a 35 for the class B push-pull amplilier circuit.v magnitude of about one or .two megohms. The

Accordingly, it is pointed out that an important Control eleerede, 01 grid, 0f the eleetlOn diS- object of the present invention is to provide a charge 'tube 1 isconnected by means of Yaf-lead push-pull amplifier circuit, embodying a gain 9 to any desired point on the resistor 6 bymak- 40 control arrangement which includes but one ading the end of the lead 9 in contact with the 40 justable element and grid circuits balancedat all ISSSOI 5 adjustable, as at l0. times with respect to ground, and whichaddi- The grid Of thetube 3 iSeOIlIl'eeted by a lead tionally includes a means for converting at will 9' to the low potential 'side of the resistor 6. the operating characteristic of l the amplifier It will benoted that both tubes 1 and 8 are of whereby the latter may function as a class A, the so-'calledpentode type. That is to say, these 45 class B or class C'fpush-pull' amplifier circuit. tubes are of the type employinga suppressor Other `objects ofthe present invention lare to grid, the latter being disposed between the posiimprove generally the simplicity and eliiciency tive screen grid and the anodeand being conof gain control devices for an audio frequency nected withinthe tube to the cathode of the tube amplier, and to particularly provide a volume whereby thev suppressor grid is maintained sub- 50 control adjustment in apush-pull ampliiiercireuit stantially at cathode potential. This type of which is not only durable and reliable in operatube is wellknown to those skilled in the art'and tion, but economically manufactured and. inneed 'notbedescribed in any further detail.` The stalled in the circuit. anodesof tubes 'I and 8 are connected to a come of the Ip vs. Eg characteristic; a class B amplimeans ef gldl'eeteatn. 25

The nov features which I believe `to be charmon transformer. primary Il),V the lmidpoint of 55 i the latter being connected to the positive terminal of a source of direct current energy I I, the

f negative terminal of the latter being grounded.

in Fig. 1. In any event, the midpoint of the resistors I2, I2 is connected by 'an adjustable lead I3 to any desired point on the resistor I4 which is connected in shunt across the source of direct current biasing potential I`5,"the'latterhaving`its positive terminal grounded.

By adjusting the lead I3 to an appropriate point on the'resistor I4 itis ypossible` to` bias the grids 'of tubes 'I and VI!v i'nsuch a manner that it can operate either as a class A, class B or class C ampliiier. In Fig. 2 there is shown the'Ip vs. Eg characteristic ofarr amplifier tube. There is designated on this characteristic the'bias required Afor class B amplifiery operation; It will be noted'that to secure such operation it is necessary to adjust the lead I3 on the resistor III to'that-point'which will' result in biasing ofthe grids of tubes I and 8 so that withM no signal applied they will be biased substantially',at` cut-oir of the characteristic. f

It will thus be seen thatwith this type of adjust-ment, plate current will only iioW when signals are impressed'upon the vcommon input circuit of the push-pull rampliiier.` This'type of operation is highly desirable in cases Where it is desired to economize plate current, as for-"example in radio receivers installed in automobiles and the like." That is to say, in casesrwhere sources of current must'be economized class B amplier operation is" desirablej Similarly, by proper adjustment of theV lead I'it` is possible to bias the grids of tubesl 1 and 8y at va point such as C in Fig. 2 thereby resulting in class C' amplier operation. Insome cases this type of operation is desirable.v

Lastly, by suitable adjustment'of the lead I3 the grids' of tubes I- and 8L can be so` biasedl that class A amplifier' operation will result, it' being well understood that such operation, takes place along/.the linear portion of the cl'iaract'eri'stic shown in Fig. 2'.

' Assuming, however, that the lead I3' has` been adjusted to render the push-pull` amplifier opera.- tive as a class B push-pull' amplifier, it will be seen that effective and simple gain control isisecured by adiustment of the singletap I Il, and that simultaneously the grid circuitstof tubes 'I and Bare maintainedbalan'ced at all times with respect to ground. Furthermore, it should be noted that parasitic oscillations arel substantiallyeliminated by thev resistors I2', I2, thus comprising another important. advantage of the present invention.

It is believed. that the operation olthe' gain 'control arrangement 6, I Il will be obvious from the aforementioned description'and the drawing. Whenever it is desired to reduce the volume of the reproducer, coupled by transformer. 20' to the output of the push-pull' amplifier, itis merely necessary to move the tap" I0 towards 'the' low potential terminal' of the resistor 6L lIt is; of

course, to be understood that any type of utilization means may be employed, a reproducer not being the only device adapted to be coupled to the output of the push-pull ampliiier. To increase the gain of the amplifier it is merely necessary to move the tap ID towards the high potential side of the resistor 6. In any case, the grid circuits of tubes 'I and 8 are maintained exactly balanced with respect to ground thus resulting in optimum push-pull operation. Simultaneously, the production of parasitics is constantly reduced, and even eliminated, by the resistorsl I2, I2'. In virtue of the power derived from the pentode tubes connected in push-pull, it is quite feasible to employ only the single pushpull stagev for audio vfrequency amplification between the detector and a reproducer.

The present'invention lends itself, in addition, to useina receiving circuit employing automatic gain control of the radio frequency amplifier stages.y It is not believed necessary to describe any such automatic gain controlarrangement since those skilled in theprior art are well acquaintedwith the type of circuit wherein a special .automaticy volume control tube is utilized in connection with the detector output for controlling the gain of the radio frequency amplifier in such a manner' that a substantially constant radio frequency amplification level is maintained` at all times, Such an arrangement is shown, for example', inv British Patent 28B-,120 of May 10th, 1928. InA such a case variation of the gain control employed `in the push-pull circuit would result in a'highly satisfactory type of volume control adjustment, particularly since the radio frequency amplification levelv would be maintained substantially constant by thel aforementioned automatic gain control.

Furthermore, While I have indicated and described one arrangement for carrying my invention'into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organization shown and described, butk that many modifications may be made Without departing from the scope of my inventionrasset forth in the appended claims.

What I claim is:

lv In av radio receiver including a detector, a push-pull ampliiier having its common input circuit coupled to the detector output, the common output circuit of the push-pull amplifier includ- Iing coupling means, said push-pull amplier comprising a pair of tubes, aresistor insaid common input circuit having one of its. terminals xedly connected to the grid of one of the pushcommon input circuit 4having oneof its terminals iixedl'y connected to the grid of one of the pushpull tubes, and manual volume controlA means adjustably connecting the grid; of the' otherv pushpullv tube to said resiston. a resistive' path having its' midpoint' groinided, said' resistive path comprising, a pfff.

between saidgridgg-I i 3. A push-pun amplifierl `circuit*tsnriiri'sine e' resistors connected in series betweenvthegrids of the push-pull tubes, said resistors being of the samemagnitude, means `for-grounding the common terminals of said resistors, said means including an adjustable source of direct current potential, and a variable resistor connected in shunt with said resistors, the grid of one of said push-pull tubes being permanently connected to one of the terminals of said variable resistor, and the grid of the remaining push-pull tube being connected to the adjusting element of the variable resistor.

4. A push-pull amplifier circuit comprising a pair of tubes -having a common input circuit adapted to be coupled to a source of energy to be amplified and a common output circuit including means for coupling the amplifier to a device for utilizing the amplified energy, a pair of grid leak resistors connected in series between the grids of the push-pull tubes, said resistors being of the same magnitude, means for grounding the common terminals of said resistors, said means including an adjustable source of direct current potential, and a variable resistor connected in shunt with said resistors, the grid of one of said pushpull tubes being permanently connected to one of the terminals of said variable resistor, and the grid of the remaining push-pull tube being connected to the adjustable element of the variable resistor, said adjustable potential source'being adapted to vary at will the operating characteristic of the push-pull amplifier in such a manner that it can be operated as a class A, class B or class C amplifier.

5. A push-pull amplifier circuit comprising a pair of tubes having a common input circuit adapted to be coupled to a source of energy to be amplified and a common output circuit including means for coupling the amplifier to a device for utilizing the amplified energy, a pair of grid leak resistors connected in series between the grids of the push-pull tubes, means for grounding the common terminals of said resistors, said means including an adjustable source of direct current potential, and a Variable resistor connected in shunt with said resistors, the grid of one of said push-pull tubes being permanently connected to one of the terminals of said variable resistor, and the grid of the remaining push-pull tube being connected to the adjustable element of the variable resistor, said grid leak resistors having such magnitudes that parasitic oscillations are substantially eliminated and the grid circuits of said push-pull tubes are maintained balanced at all times with respect to ground.

6. In a radio receiver, an audio frequency amplifier, an audio frequency transformer, said amplier including a pair of tubes connected in push-pull, a manual volume control device for said receiver comprising a fixed resistor connected across the secondary of said transformer, an adjustable connection from the grid of one of the amplifier tubes to said resistor, a connection from the grid of the other amplifier tube to one terminal of the resistor, an impedance in parallel with said resistor, and a connection from r {df f griffeak resistere connected' saidimpdant'e-'tov4 the fcathodes vof said lpair', for. tubes fui n, 11. .k ,21' 1s z- '1.4 In-a radio receivenfan-'audio-frequency' am; pli'fler, an audio frequency transfrmerglsaid amy-`4 plifier'includingj a 'pair o'f peiitode#poweroutrr` put tubes connected iny push-pull; a` manual -volume control-jdevice vfor vSaid receiver comprising a' fixed; resistorfconne'ctedy across l'the secondary. of said l transformer; i an impedance l in 'parallel with at least-apart of said' resistor, connections from saidfimpedance -to Athe cathodes of 'said joutput tubes, an adjustable connection from the grid of one of the amplifier tubes to said resistor, and a fixed connection from the grid of the other amplifier tube to one terminal of the resistor.

8. In a radio receiver, a detector stage, an audio frequency amplifier, an audio frequency transformer having its primary connected across the output electrodes of the detector tube, said amplifier including a pair of tubes connected in push-pull, a. manual volume control device for said receiver comprising a fixed resistor connected across the secondary of said transformer and an adjustable connection from the grid of one of the amplifier tubes to said resistor, the grid of the other amplifier tube being fixedly connected to one terminal of the resistor and means for maintaining the grids of the amplifier tubes balanced at all times with respect to ground.

9. In a radio receiver, a detector stage, an. audio frequency amplifier, -an audio frequency transformer having its primary connected across the output electrodes of the detector tube, said amplifier including a. pair of tubes connected in push-pull, a manual volume control device for said receiver comprising a fixed resistor connected across the secondary of said transformer and an adjustable connection from the grid of one of the amplifier tubes to said resistor, the grid of the otheramplifier tube being fixedly connected to one terminal of the resistor, a resistance path connected between the amplifier grids, and a connection to ground from a point on said path ldividing the latter into resistors of equal magnitude.

10. In a radio receiver, a detector stage, an audio frequency amplifier, an audio frequency transformer having its primary connected across the output electrodes of the detector tube, said amplifier including a pair of pentode power output tubes connected in push-pull, a manual volume control device for said receiver comprising a xed resistor connected across the secondary of said transformer and an adjustable connection from the grid of one of the amplifier tubes to said resistor, the grid of the other amplifier tube being fixedly connected to one terminal of the resistor, a resistance path connected between the amplifier grids, and a connection to ground from a point on said path dividing the latter into resistors of equal magnitude, the cathodes of said amplifier tubes being grounded.

11. In a radio receiver, a detector stage, an audio frequency amplifier, an audio frequency transformerY having its primary connected across the output electrodes of the detector tube, said amplifier including a pair of tubes connected in push-pull, a manual volume control device for said receiver comprising a xed resistor connected across the secondary of said transformer and an adjustable connection from the grid of one of the amplifier tubes to said resistor, the grid of the other amplifier tube being fixedly conpedance connected in parallel with at least a part ofl the rst mentioned impedance, an adjustable connectionfromthe rigid of one of the'amplier to one of said impedlances, a. connection from the *grid of the other ampler tube to one terminalaoif one of.'A the impedances, a connection fromoneof seid. impedance: to the cathodes' of said, temes;v and, means biasing` the'grids of said tubes included in` thevllastmentined connection.

y STQDDARD BARDEN. 

